Means for continuously cooling powder produced by granulating a molten material

ABSTRACT

A powder is produced by atomizing molten material by means of a jet of an atomizing fluid. The atomizing zone is surrounded by a wall defining a comparatively narrow passageway, to the effect that the jet of atomizing fluid will act as a jet pump creating a circulation of the gas in the atomizing chamber. Means are provided for cooling said gas.

United States Patent [1 1 Hellman et a1.

[451 Nov. 13, 1973 MEANS FOR CONTINUOUSLY COOLING POWDER PRODUCED BYGRANULATING A MOLTEN MATERIAL Inventors: Per Hellman, Soderfors; ErikAnders Ake Josefsson, Borlange, both of Sweden Assignee: StoraKopparbergs Bergslags Aktiebolag, F alun, Sweden Filed: Dec. 5, 1972Appl. No.: 312,379

Related Application Data [56] References Cited UNITED STATES PATENTS2,284,023 5/1942 Scripture 264/12 X 3,428,718 2/1969 Helin et al.3,588,951 6/1971 l-legmann 425/7 Primary ExaminerRobert D. BaldwinAtt0meyHenry W. Koster [57] ABSTRACT A powder is produced by atomizingmolten material by means of a jet of an atomizing fluid. The atomizingzone is surrounded by a wall defining a comparatively narrow passageway,to the efiect that the jet of atomizing fluid will act as a jet pumpcreating a circulation of the gas in the atomizing chamber. Means areprovided for cooling said gas.

1 Claim, 2 Drawing Figures Division of Ser. No. 97,991, Dec. 14, 1970,abandoned.

Foreign Application Priority Data Dec. 15, 1969 Sweden 17286/69 US. Cl.425/7, 264/12 Int. Cl B22d 23/08 Field of Search 425/7;

PATENTEDNUY 13 1975 377L929 SHEET 20F 2 Fig. 2

1 MEANS FOR CONTINUOUSLY COOLING POWDER PRODUCED BY GRANULATING A MOLTENMATERIAL This is a divisional of Application Ser. No. 97,991 filed Dec.14, 1970, now abandoned.

The present invention relates to apparatus for manufacturing powder byatomizing a molten material in A which a tapping stream or jet of themolten material is atomized or broken into fine drops when it comes intocontact with an atomizing agent, normally a fluid, which is directedunder high pressure in the form of jets against the stream of moltenmaterial.

The requirements of a powder vary with the field of use. The basicproperties of the powder are determined by its chemical composition, thedistribution of particles of different sizes and the shape andmicrostructure of the particles. The chemical composition is dependenton the composition of the original material and any oxidation orreduction of this material during the pulverizing process itself. Thesize and shape of the powder particles are substantially dependent onhow atomization of the molten material is carried out, whereas themicrostructure is to a great extent dependent on how the drops obtainedduring atomization are cooled. The shape of the powder particles is alsodependent on how the drops are cooled since drops which knock against ahard object before they have had time to solidify will be deformed. Inthe process of manufacturing a powder by atomizing a molten material,therefore, the powder is usually collected at the bottom of a bath ofcoolant, normally consisting of water. However, such water baths causethe powder obtained to be oxidized on the surface and in manycases it istherefore desirasble to cool the drops while falling freely in some sortof inert atmosphere until they have completely solidified and cooled sothat there is no longer any risk of the particles being deformed orsticking together. When a material having a high melting point, forexample metal, is atomized the quantity of heat which must be removedbefore the drops have solidified is quite considerable, and since theparticles furthermore may not come into contact with any solid objectduring the solidification process, the distance which the drops mustfall freely before they solidify is relatively long if no other stepsare taken. This means that the atomization chamber in which theatomization takes place must be made extremely high.

The present invention relates to means for accelerating the cooling ofpowder manufactured by the atomizing of a molten material. The inventionis principally intended for use in the manufacture of powder fromalloyed steel for producing compact steel by sintering the powder underpressure. Of course, the invention may also be used in all other caseswhere powder is produced by atomizing a molten material. One of thereasons which makes the method and means according to the inventionparticularly suitable for the manufacture of powder from alloyed steelis that in the manufacture of such powder it is extremely important thatthe powder is entirely free from oxide as it is not usually possible todeoxidize the powder before it is used. (Alloyed steel usually containsalloying elements which form extremely stable oxides which are verydifficult to reduce). Powders which are to be used for thepressuresintering of powder bodies should also have spherical particleswith a smooth surface without bubbles or cavities. A spherical particleshape simplifies the sintering process, since compact powder bodies canbe obtained by means of a relatively simple pre-pressing process, andthese bodies can then be sintered to form compact steel bodies.

When manufacturing metal powder by atomizing a molten material, a streamof the molten metal is usually disintegrated by directing one or morejets of some suitable atomizing agent, normally a fluid, for example agas or liquid or a mixture of gas and liquid, under high pressure and atan acute angle against the stream of molten metal so that this is splitup into fine metal particles or drops which are collected after theyhave been cooled to such an extent that they have solidified and reachedsuch a temperature that there is no longer any risk of the metalparticles sticking together. Generally, the jets of atomizing agent areaimed from several sides against the stream of molten metal so that allthe jets intersect each other at substantially the same point. However,it is extremely difficult to get several jets of atomizing agent tointersect the stream of molten metal at the same level. Usually,therefore, the stream of molten metal will come into contact with one ofthe fluid jets immediately adjacent their point of intersection. It hasbeen found that this kind of poor centering of the jets has an extremelynegative effect on the quality of the powder obtained, and on theatomizing process, but that a considerable improvement of thedisintegration of the molten material can be achieved if the stream ofmolten material is first intersected by a thin, sharp fluid jet, widerthan the stream of molten material, and having such high kinetic energythat it forces the stream of molten material to alter direction andspread out into a layer on top of the fluid jet, and then when thedirection of the melt has thus been altered, it is intersected byanother fluid jet having greater width than the metal layer obtained, sothat the melt is split up into free droplets. This method is furtherdescribed in our Pat.

application Ser. No. 94,148 filed Dec. 1, 1970 assigned to a commonassignee.

According to the present invention the jets of atomizing fluids are usedto effect an internal gas circulation in the atomizing chamber so thatthe cooling of the molten particles is accelerated. It is proposed thatthe top of the atomizing chamber is shaped as a jet pump, so that themovement of the fluid jets causes a suitable coolant, preferably aninert gas, to circulate throughthe atomizing chamber. The atomizingchamber is kept suitably filled with the coolant, which may for examplebe argon, and, if being continuously cooled by being brought tocirculate through a heat-exchanger, this coolant will considerablyaccelerate the solidification and cooling of the drops. When this methodis used, the

atomizing chamber can be built considerably shorter without its generalconstruction becoming much more complicated. In its most simple designthe invention requires no additional supply of energy besides thatnecessary for circulation of a suitable cooling agent through theheat-exchanger. Of course, the gas circulation in the atomizing chambercan be further improved by introducing a circulation pump in the system,or a circulation pump may be inserted parallel to the gas circulationobtained by means of the jet pump. The atomizing chamber can be madeeven shorter if the lower part is provided with a fluidized bed wherethe cooling of the particles can be finalized. In this case, the dropsneed only have solidified on the surface when they reach the fluidizingbed. It is also possible to allow the drops, after solidification, toslip or slide along an inclined cooling surface where the final coolingtakes place. The inclined cooling surface may, of course, consist ofcoiled tubing which is surrounded by some suitable coolant.

The apparatus according to the invention will be further described withreference to the accompanying drawings and the invention will be definedin the following claims.

FIG. 1 shows a cross section of an apparatus according to the invention,whereas FIG. 2 shows a modification of said apparatus.

The device shown in FIG. 1 consists of anatomizing chamber 1, providedwith an outer cooling jacket 2 and inner cooling jackets orheat-exchangers 3. A suitable coolant, for example water, flows in thecooling jackets. The cooling jacket 2 is provided with an inlet 28 andan outlet 29 for the coolant, while the cooling jackets 3 has hotcoolant inlets 30 and outlets 31. At the upper part of the atomizingchamber 1 a tundish 4 is arranged, provided with a tapping hole 5 at thebottom, through which a tapping stream 6 of the molten material, forexample the metal to be atomized, falls down into the chamber 1. On eachside of the tapping hole 5, opposite to each other, are two slitorifices 7 and 8. The orifices extend in a plane substantially at rightangles to the plane of the figure. The orifice 7 directs a flat jet 9 ofa suitable fluid, for example argon, at an angle of about 45 against thetapping stream 6. This first fluid jet forces the stream of moltenmaterial to alter direction and also to a certain extent, splits themelt in the stream of molten material into drops. The molten materialthus spreads out to form a layer on top of the fluid jet. The stream ofmolten material is then intersected by a second fluid jet 10 from thenozzle 8 at such a distance from the intersection between the stream ofmolten material and the first fluid jet 9 that most of the moltenmaterial has time to alter direction. The second fluid jet, which issubstantially parallel to the original direction of the tapping stream,completes the separation of the molten material into drops and spreadsthis as a shower 1] in the chamber 1. The drops are cooled during theirfree fall through the chamber and are collected in a fluidized bed 12 atthe lower end of the chamber 1. The fluidized bed is maintained by theaddition of argon through a number of gas inlets 13 in the lowermostpart of the chamber 1. The drops of moltem material which havesolidified to powder are fed continuously out of the chamber through theoutlet 14, through which excess argon can also be removed. The level ofthe fluidized bed is thus maintained con stant.

Between the two cooling jackets 2 and 3 an annular channel 15 is formedwhich is in communication with the lower part of the chamber 1 at 16immediately above the fluidized bed 12, and which is in communicationwith the upper part of the chamber 1 at 17 in the vicinity of theintersections of the jets 6, 9 and 10 with each other. A throttlingflange 18 is arranged around the intersections so that this part of thedevice will operate as a jet pump in which, by ejector action, themovement of the fluid jets will also draw the atmosphere existing in thechannel 15. The atmosphere of the chamber 1, which preferably consistsof some inert gas, preferably argon, will therefore circulate throughthe inside of the chamber 1 and back through the channel 15. Since thechannel 15 runs between the two cooling jackets 2 and 3 it will alsoserve as a heatexchanger. Consequently, because of the special shape ofthe upper part of the chamber 1, warm gas will be drawn away from thelower part of the chamber while cold gas is constantly supplied to theupper part. The cooling of the drops of molten material produced willtherefore be greatly accelerated and the chamber 1 can be madeconsiderably shorter than would otherwise be necessary for the drops tobe able to solidify during their free fall. The channel orheat-exchanger 15 may of course consist of one or more heat-exchangersarranged completely outside the chamber 1. The fluidized bed 12 is notabsolutely necessary but may be useful in resulting in a shorterapparatus, because the drops need only have solidified on the surfacewhen they reach the fluidized bed. As an example of the importance ofthe height of the atomizing chamber, as far as costs are concerned, itmay be mentioned that the height of the atomizing chamber for atomizinghighspeed tool steel should be about eight meters if the drops are onlycooled during their free fall without any special arrangements. With adevice according to the invention, this height can be considerablyreduced, thus resulting in savings in cost.

The device shown in FIG. 2 is a modification of the device according toFIG. 1 and those parts which are the same in both figures have thereforebeen given the same reference characters.

However, in FIG. 2 some of these common reference characters have beenomitted in order to emphasize the essentials of the figure. The moltenmaterial flowing from the tundish 4 is atomized in accordance with themethod described in connection with FIG. 1, but the drops which havesolidified to form powder particles are not finally cooled in afluidized bed. Instead they are collected on the inclined and cooledbottom plane 19 of the chamber 1 from where they are distributed overadditional inclined cooling surfaces 20 and 21 along which the powderparticles may slide until they have been cooled to such an extent thatthey can be collected at the outlet 22 without there being any risk ofthe particles sticking together. The powder is removed through a screwconveyor 32. Since the cooling surfaces 20 and 21 are inclined the riskof the powder particles being deformed is reduced, and since the finalcooling takes place while the particle is still moving along thesecooling surfaces, there is no risk of the particles sticking togethereven at temperatures at which this might otherwise be a big problem. Thebottom surface 19 of the chamber 1 is cooled by the lower part of thecooling jacket 2. The cooling surfaces 20 and 21, which in the exampleshown in FIG. 2 may consist of tubes, may either be provided withseparate cooling jackets or surrounded by, for example, a circulatingcooling medium. The inclined cooling surfaces may be designed in manydifferent ways and the example illustrated consisting of a number oftubes 20 and 21 is only one embodiment.

In order to increase the internal gas circulation in the chamber 1 tocool the drops, a part of the atmosphere is removed from the chamberthrough conduits 23, the atmosphere in this case suitably consisting ofan inert gas, for example argon, at the lower part of the chamber andcompressed by pumps 24 to a higher pressure. The gas is then returnedunder this higher pressure through the conduits 25 to the upper part ofthe chamher 1 in the vicinity of the throttling flange 18. The conduits25 are provided with outlet valves 27 for excess argon. Excess argon canalso be removed from the chamber through the valve 32. The gascirculation through the chamber 1 can be considerably increased in thisway in spite of the fact that the energy supplied to the pumps 24 isrelatively small. The movement of the fluid jets is used at the sametime for circulating the gas through the channel 15. If desired, thecirculation pumps may be placed in the channel or its extension and not,as in this example, parallel with the channel 15. If the gas beingforced to circulate is supplied under pressure, care must of course betaken to see that this does not affect the atomizing processunfavourably. The gas being forced to circulate, may also be supplied atthe same pressure as that prevailing inside the chamber, but thisrequires some alterations of the arrangement shown in FIG. 2. In orderto cool the circulating gas the conduits may be provided with coolingjackets 26.

The means according to the invention is not limited to what is describedin connection with these drawings, but can be varied in accordance withthe basic idea of the invention,

What we claim is:

l. A device for converting molten metal to a fine metal powder whichcomprises an atomizing chamber having an upper portion for atomizing astream of mol- 6 ten metal into droplets, a lower portion for collectingand discharging the metal powder, and an intermediate portion forcooling and solidifying said droplets, the intermediate portion of saidchamber comprising an outer cooling jacket forming an exterior wall andan inner cooling jacket spaced from and within the outer cooling jacketto form an annular passage for upward movement of cooling gas and aninner passage for downward movement of solidifying metal droplets, theupper portion of said chamber having an inlet opening for the moltenmetal to form a continuous and descending stream of molten metal, afirst slit orifice below and to one side of said inlet for delivering afirst flat jet of argon gas to split the stream of molten metal into alayer of droplets, said first jet having a width exceeding the width ofthe stream of molten metal and intersecting said stream at an angle ofabout 45, a second slit orifice to the other side of the inlet fordelivering a second flat jet of argon gas to intersect and completeatomization of said stream of molten metal, said second jet beingsubstantially parallel to the initial stream of molten metal and beingdirected into the inner passage of the intermediate portion of thechamber to effect downward movement therethrough of solidifying metaldroplets and argon gas and upward movement of said gas through theannular passage.

1. A device for converting molten metal to a fine metal powder whichcomprises an atomizing chamber having an upper portion for atomizing astream of molten metal into droplets, a lower portion for collecting anddischarging the metal powder, and an intermediate portion for coolingand solidifying said droplets, the intermediate portion of said chambercomprising an outer cooling jacket forming an exterior wall and an innercooling jacket spaced from and within the outer cooling jacket to forman annular passage for upward movement of cooling gas and an innerpassage for downward movement of solidifying metal droplets, the upperportion of said chamber having an inlet opening for the molten metal toform a continuous and descending stream of molten metal, a first slitorifice below and to one side of said inlet for delivering a first flatjet of argon gas to split the stream of molten metal into a layer ofdroplets, said first jet having a width exceeding the width of thestream of molten metal and intersecting said stream at an angle of about45*, a second slit orifice to the other side of the inlet for deliveringa second flat jet of argon gas to intersect and complete atomization ofsaid stream of molten metal, said second jet being substantiallyparallel to the initial stream of molten metal and being directed intothe inner passage of the intermediate portion of the chamber to effectdownward movement therethrough of solidifying metal droplets and argongas and upward movement of said gas through the annular passage.